It has been known that when a thermosetting synthetic resin (hereinafter, simply referred to as resin) is cured, the resulting resin can be made into an article exhibiting excellent corrosion resistance, chemical resistance, water resistance, mechanical properties, etc. Examples of such a resin are a vinyl ester resin, an urethane(meth)acrylate resin, an unsaturated polyester resin, a polyester(meth)acrylate resin, acrylic syrup, etc. The "thermosetting" referred in the present invention means the properties of the resin that it is cured with heating or irradiation of an active energy beam.
For example, the vinyl ester resin is generally produced by reacting an epoxy compound with an unsaturated mono-basic acid with the use of an esterification catalyst. However, since these resins and materials thereof contain reactive groups, such as vinyl groups, these resins have a problem that they readily turn into gel as the reactive groups bond to one another during the producing step or the storage.
It is known that oxygen is an effective component to prevent such unwanted gelation of the resins. Therefore, in order to prevent the gelation during the producing step, storage, or transportation, preventive measures are taken, for example, flowing a dry air through these resins, opening the container periodically, and reducing the filling rate of the resins in the storage container.
However, a part of these resins turns into a colored compound as it undergoes oxidation with oxygen flown through the resins during the reaction. Thus, the resulting resins are colored considerably. For example, their coloring degrees are 3-8 in the Gardner color scale. Moreover, even if a sufficient amount of air coexists with these resins at all times, this is not a satisfactory preventive measure against the gelation of these resins, and these resin still have a greater possibility of turning into gel during the producing step, storage, or transportation.
Various kinds of methods are discussed to prevent the gelation. For example, Japanese Laid-open Patent Application No. 107090/1977 (Tokukaisho 52-107090) discloses a method of stabilizing stored thermosetting resins by adding a living radical compound. The thermosetting resins referred therein are defined as a product obtained by dissolving a reaction product of an epoxy resin with acrylic acid and/or methacrylic acid into a polymeric unsaturated monomer.
However, the process disclosed in the above publication has a problem that the living radical compound is highly toxic and must be handled with care.
Adding well-known polymerization inhibitors, such as hydroquinone, p-methoxyphenol, t-butyl catechol, and phenothiazine, to these resins can improve the storage stability, and is known as another method for preventing the gelation.
However, the above-detailed well-known polymerization inhibitors are colored products or color the resulting thermosetting resin composition eventually. In other words, while the thermosetting resin composition is kept open to air, the polymerization inhibitors undergo oxidation with oxygen dissolved into the thermosetting resin composition and turn into a colored compound, thereby coloring the thermosetting resin composition.
Furthermore, if more than one kind of polymerization inhibitor is used, or a large amount of the polymerization inhibitor is used to further improve the storage stability, the thermosetting resin composition tends to take a longer time to turn into gel when being cured, thereby deteriorating the curing property of the resin.
Although these resins render excellent properties, the use of the same is limited because they are readily colored. They are used as materials for FRP (fiber reinforced plastic) articles, molding articles, lining, coating, etc., only when the outward appearance is not a great concern. In other words, these resins are not suitable for the use in a bath tub, a washstand counter, a gel coat, etc. where the outward appearance is of a great importance, or for the use as an active energy beam-curable resin when an excellent active energy beam-curing property is required.
To solve the above problem, Japanese Examined Patent Publication No. 34771/1991 (Tokukouhei 3-34771) discloses a process of producing a light-color vinyl ester resin in which the coloring is suppressed. More specifically, an epoxy compound, an unsaturated mono-basic acid, and an unsaturated poly-basic acid are reacted with one another using an esterification catalyst in the presence of triphenylstibine under an inert atmosphere.
Also, Japanese Examined Patent Publication No. 23232/1994 (Tokukouhei 6-23232) discloses a process of reacting an epoxy compound, an unsaturated mono-basic: acid, and an unsaturated poly-basic acid with one another using an esterification catalyst in the presence of phosphorous acid and/or diester phosphite.
However, when the vinyl ester resin is produced by the above process, or when the above process is adopted to produce an urethane(meth)acrylate resin or a polyester(meth)acrylate resin, a large amount of addition agents, such as triphenylstibine, phosphorous acid, and diester phosphite, must be used to trigger the reaction under an inert atmosphere or an oxygen lean atmosphere to confer a significant light-color property to the resulting resin. Thus, there is a problem that the materials and a reaction product readily turn into gel.